In the image-forming process according to electrophotography, an electrostatic latent image is formed on an inorganic photosensitive material of selenium, a selenium alloy, cadmium sulfide or amorphous silicon, or on an organic photosensitive material comprising a charge generating material and a charge transporting material, developed with a toner, transferred and fixed to a paper sheet or a plastic film to give a visual image thereon. The photosensitive material is chargeable positively or negatively depending upon the constitution thereof. In a case wherein an image area remains as an electrostatic latent image after the exposure to light, a toner which is chargeable with a polarity reverse to that of the photosensitive material is used in the development of the latent image, while in a case wherein the charge in an image area is erased to conduct reversal development, a toner which is chargeable with the same polarity as that of the photosensitive material is used in the development of the latent image.
Although a toner is substantially constituted of a binder resin, a coloring material and additives, a charge control agent is generally also added to the toner in order to impart desirable frictional electrification properties (charge rate, charge level or charge stability), long-term stability and environmental stability thereto. The characteristics of the toner are greatly affected by the charge control agent. In a case wherein the development is conducted with a positively chargeable photosensitive material and a negatively chargeable toner, or in a case wherein the reversal development is conducted with a negatively chargeable photosensitive material, a negatively chargeable toner is used and a negatively chargeable charge control agent is incorporated thereinto.
Further, the charge control agent to be used for a color toner must be colored so lightly as not to exert any influence on the color of the toner, and preferably is colorless. Such a light-colored or colorless charge control agent includes metal complex salts of hydroxybenzoic acid derivatives as described in Japanese Examined Patent Publication No. 42752/1980 and Japanese Unexamined Patent Publication Nos. 69073/1986 and 221756/1986; metal salts of aromatic dicarboxylic acids as described in Japan Unexamined Patent Publication No. 111541/1982; metal complex salts of anthranilic acid derivatives as described in Japanese Unexamined Patent Publication Nos. 141453/1986 and 94856/1987; organoboron compounds as described in U.S. Pat. No. 4767688 and Japanese Unexamined Patent Publication No. 306861/1989; and biphenol compounds as described in Japanese Unexamined Patent Publication No. 3149/1986.
However, some of these charge control agents are chromium compounds which are in danger of causing environmental pollution, some of them are unavailable as completely colorless ones' and some of them each have a disadvantage of a low charge imparting effect, generating a large amount of a reversely charged toner, poor dispersibility or poor stability. Thus, no charge control agent exhibiting satisfactory performances has been found as yet.